Campanula or bell flower has awesome purple, or pink flowers. Sold practically everywhere that sells plants, this perennial will quickly fill a large space and most likely seed to other open spaces in your gardens and fill those too. Highly invasive and trouble some, spreads also by runners. These roots may go far as 3″, no wait 3′!!! into your soil.
Eradication is almost impossible as it creeps into the clumps of any other perennials in your garden beds. The best way to deal with this garden beast is to dig it out, or spray with round-up or weedbgone as soon as you see it ! If your neighbors have it, you will be dealing with this forever. Good news is that if you keep pulling it and spraying it with chemicals one day it might be manageable.